The invention relates to an expansive sleeve for applying precisely centered expansion stress to a workpiece, by means of outward radial expansion brought about by axial pressure, said sleeve comprising outwardly open grooves and inwardly open grooves, which grooves are delimited by encircling circumferential web members and cylindrical circumferential stress covers, at least one of which stress covers is associated with the workpiece. Each such workpiece-associated stress cover is provided with thickened regions which are basically cylindircal and which are provided with stress surfaces delimited by grooves, wherewith the stress cover itself forms the bottom of said grooves, and wherein the radial thickness of the thickened regions is at least one-half greater than the thickness of the stress cover.
In a known expansive sleeve of this type (Swiss Pat. No. 309,260), three encircling thickened regions are provided per stress cover (and per workpiece) each of which comprises a thread which forms a stress surface having a sharp angular tooth configuration which extends to the base of the said thread. Apart from this threaded configuration, the general shape of the known expansive sleeve is such that the stress cover itself comprises a cylindrical stress surface. The expansion of this type of expansive sleeve depends upon the fact that the stress cover bows under pressure; hence, the first cylindircal stress surface bows under pressure, whereby the locus of contact with the workpiece is only a circular arc.
There are various disadvantages associated with this limited circular contact locus of the outwardly bowed stress surface. The stress surface is in actual use primarily only along the line of contact. In the case of a short expansive sleeve with only one stress surface acting on the workpiece, it is thus possible for the workpiece to bend around the peak of the bowed stress cover upon which it sits. In the case of an expansive sleeve arrangement employing two or more such stress surfaces acting on the workpiece, the workpiece must be positioned so precisely that its two end sections are forced to rest on the respective outwaredly bowed portions of the two outer stress surfaces. In particular, the workpiece must not overextend either of these peak loci unless auxiliary end extensions are also provided therefor.